1. Field of the Invention
The present invention relates to a tractor having a power transmission mechanism, especially to a tractor having a power transmission mechanism constructed so that a drive sprocket is arranged in a front portion of a crawler traveling unit of a crawler tractor and power is transmitted from an engine to the drive sprocket.
Furthermore, the present invention relates to a tractor having a differential mechanism provided in a traveling vehicle such as a crawler tractor.
Moreover, the present invention relates to a construction of a hydraulic circuit of a traveling vehicle of a tractor. In more detail, the present invention relates to a drain oil passage of a hydraulic circuit of a tractor constructed so that power from an engine is transmitted to a hydraulic clutch type forward/rearward traveling switching device and a main transmission, subsequently transmitted to a sub transmission and changed in speed, and then transmitted to the turning HST so as to control traveling.
Besides, the present invention relates to an art of a track roller of a crawler traveling unit of a tractor so as to support a crawler belt provided in the crawler traveling unit.
2. Description of the Prior Art
Conventionally, there is well known a tractor which is a traveling vehicle having a crawler traveling unit (crawler tractor) for various kinds of agricultural works on a field. The tractor is constructed so that an engine and a front axle casing are arranged in the front portion of the vehicle, and a transmission casing is arranged in the rear portion of the vehicle. Power from the engine is changed in speed in the transmission casing, and then transmitted from an output shaft projected from a power take-off part of the front portion of the transmission casing through a power transmission shaft to an input shaft provided in the rear portion of the front axle casing so as to drive the crawler traveling unit through a differential housed in the front axle casing (for example, see the Patent Literature 1).
With regard to the conventional crawler tractor, the height of the output shaft of the transmission casing is different from that of the input shaft of the front axle casing. Accordingly, the output shaft and the input shaft are connected through universal joints and a propeller shaft so that power can be transmitted though the power transmission shaft. However, with regard to a large tractor having a high-powered engine, the transmission torque also becomes large. Accordingly, the universal joints and the propeller shaft must be enlarged so that the weight thereof is also increased, whereby the attachment work of the power transmission shaft becomes difficult and the cost is increased. Furthermore, the kind of the universal joints to be used is limited, whereby the design is restricted.
Conventionally, with regard to the traveling vehicle traveling by the crawler traveling unit, there is well known a differential mechanism that power from the engine is changed in speed in the transmission, and then inputted to a differential and transmitted to left and right output shafts so as to perform straight travel, and a turning HST is driven by operating a steering wheel and power from the turning HST is transmitted to the differential so as to generate difference of rotation speed between the left and right output shafts, thereby turning travel is performed. With regard to such a differential mechanism, power outputted from the turning HST is adjusted corresponding to operation amount of the steering wheel so as to change the difference of rotation speed between the left and right output shafts, thereby adjusting the turning radius of the vehicle at the time of turning (for example, see the Patent Literature 2).
As mentioned above, with regard to the conventional differential mechanism, power outputted from the turning HST is adjusted corresponding to operation amount of the steering wheel. However, for turning widely by small operation amount in the case of low speed traveling and turning slightly by large operation amount in the case of high speed traveling, or for reducing speed at the time of turning, a complex linkage mechanism is provided between the steering wheel and a speed change shaft of the turning HST or a main speed change lever. As a result, the number of parts increase and assembly work becomes troublesome. Power outputted from the turning HST causes large output loss, whereby the power cannot be used maximally.
Conventionally, with regard to a turning vehicle such as a tractor, power from the engine is transmitted to a hydraulic clutch type forward/rearward traveling switching device and a main transmission, subsequently transmitted to a sub transmission and changed in speed for driving the traveling unit, and then transmitted to the turning HST so as to control traveling, a hydraulic pump is provided so that pressure oil stored in the transmission casing as a pressure oil tank is pressingly sent to each hydraulic device, and the hydraulic pump is driven by the engine. With regard to a hydraulic circuit of such a tractor, pressure oil from the hydraulic pump is cooled by an oil cooler and then supplied to the turning HST. The pressure oil returning from the turning HST is pressingly sent to the hydraulic clutch of the forward/rearward traveling switching device for lubrication, and then returned to the transmission casing (for example, see the Patent Literature 3).
With regard to the conventional tractor, pressure oil cooled by the oil cooler is supplied to the turning HST, and the pressure oil returning from the turning HST is supplied to the hydraulic clutch of the forward/rearward traveling switching device for lubrication. Accordingly, the cooling efficiency is low and the transmission cannot be cooled efficiently. The pressure oil is supplied to the hydraulic clutch, whereby loss of oil pressure is large and sufficient cooling effect cannot be obtained.
Now, conventionally, with regard to the crawler traveling unit, a track roller is rotatably provided between a drive sprocket, transmitting power from a drive source such as the engine to the crawler belt, and an idler (driven sprocket) provided oppositely to the drive sprocket in the traveling direction.
Then, the track roller is provided so as to support the crawler belt wound around the drive sprocket and the driven sprocket, to support the slack of the crawler belt, and to make the transfer of the crawler belt smooth.
By constructing an equalizer rotatably supporting front and rear pair of track rollers on a track frame, large shaking of the vehicle body at the time of riding across a convexity on the field is suppressed.
An example of the crawler traveling unit having such a construction is shown in later-discussed Patent Literature 4.
Concretely, the track roller of the crawler traveling unit is rotatably provided on a support member fixed to a frame or the like of the crawler traveling unit.
In this case, when vibration is caused by unevenness of the traveling surface (field surface) or lag of the crawler belt at the time of traveling for example, the vibration is transmitted from the track roller through the track frame to the vehicle body frame.
As a result, not only is a bad influence exerted on the vehicle but also the vibration shakes the whole crawler traveling unit.
Accordingly, an operator operating a crawler tractor or the like to which the above-mentioned crawler traveling unit is adopted may be interfered with operating and be tired by the vibration.
Then, it is necessary to provide a tractor having a track roller of a crawler traveling unit so as to suppress vibration transmitted from the track roller to the crawler traveling unit and to reduce load on an operator of the crawler tractor having the crawler traveling unit for enabling comfortable operation.    Patent Literature 1: the Japanese Patent Laid Open Gazette 2001-253362    Patent Literature 2: the Japanese Patent Laid Open Gazette 2003-2228    Patent Literature 3: the Japanese Patent Laid Open Gazette 2001-55161    Patent Literature 4: the Japanese Patent Laid Open Gazette 2002-316661